New complexes with the general formula [RuCl(CO)(dppb)(diimine)]PF6, [dppb = 1,4-bis(diphenylphosphino)
butane; diimine = 2,20-bipyridine (bipy) or 1,10-phenanthroline (phen)], were prepared. Thus, the
complexes ct-[RuCl(CO)(dppb)(bipy)]PF6 (1), ct-[RuCl(CO)(dppb)(phen)]PF6 (2), tc-[RuCl(CO)(dppb)(bipy)]PF6
(3), tc-[RuCl(CO)(dppb)(phen)]PF6 (4), cc-[RuCl(CO)(dppb)(bipy)]PF6 (5) and cc-[RuCl(CO)(dppb)(phen)]PF6
(6) were obtained and characterized. In this case, the first letter in the prefixes indicates the position of CO
with respect to the chlorido ligand and the second one is related to the phosphorus atoms. The compositions
of the complexes were confirmed by analytical techniques and an octahedral environment around the
ruthenium was confirmed by single-crystal X-ray diffraction of the complexes ct-[RuCl(CO)(dppb)(bipy)]PF6 and
cc-[RuCl(CO)(dppb)(phen)]PF6. The oxidation potentials of the complexes were determined by cyclic
voltammetry and it was found that they vary according to the CO position in the complexes. In order to obtain
information on the stability of the ct, tc and cc-[RuCl(CO)(dppb)(bipy)]PF6 (1), (3) and (5) isomers, computational
studies were carried out, and they showed large differences between the HOMO/LUMO energies. As
monitored by 13C NMR, the stability of the complexes with respect to CO displacement, for at least 72 h, in
DMSO-d6 solution, is independent of the CO position in the complexes. Pharmacological evaluation of the
complexes against the Trypanosoma cruzi parasite revealed the structure–activity relationships, showing that
the presence and position of the CO ligand in the complexes are relevant for the antiparasitic activity of the
compounds. The most active compound, the tc-[RuCl(CO)(dppb)(bipy)]PF6 isomer, presented potent
antiparasitic activity, which was achieved by causing oxidative stress followed by parasite cell death through
necrosis. Thus, the findings presented here demonstrate that the use of a carbonyl ligand provides stability and
pharmacological properties to ruthenium/diphosphine/diimine complexes.